I think you're right re: Spike/Buffy vs. Angel/Buffy. The S/B relationship has a stronger foundation by the end of the series because they have gotten to know and understand one another on a deeper level. That was one of the things I really liked about S7 was the understanding that eventually developed.

My major problem with the idea of passion as so much of Hollywood, etc. portrays it is that passion is the be-all and end-all. Passion, because it isn't made to last, sets people up for failure. I've seen this while doing counseling and with friends whose expectations that they're going to have this great romantic love that lasts into all eternity isn't met because it's not a realistic expectation.